Enhanced laser cleaning via direct line beam irradiation

A fluence advantage was achieved in dry/damp laser cleaning by reduction of the laser beam dimensions. 0.1 m Al₂O₃ particles were removed from glass slides using a KrF excimer laser (248 nm). As the width of the rectangular beam was reduced, a decrease in the threshold fluence required for particle removal was observed. Modelling based on the simplified thermal–mechanical response of the substrate and particle to the laser pulse does not describe the experimental results presented here. The case of dynamic expansion must be further considered, but it is believed that other mechanisms are involved in a full explanation. PACS 42.62.Cf     

Ablative thresholds in laser cleaning of substrates from particulates

Laser cleaning thresholds based on the local ablation of substrate material are studied theoretically. Results are compared with the experimental data on the cleaning of silicon wafers from spherical silica particles using laser wavelengths at 248, 532 and 1064 nm. Calculations take into account local enhancement in the laser-light intensity and are based on analytical solutions for the temperature distribution. Influence of vapor atmosphere on cleaning thresholds is studied experimentally and theoretically. Here cleaning is assisted by explosive vaporization of capillary condensed water. A possibility to increase the window for damage-free cleaning by varying the pulse duration and laser wavelength is also discussed. PACS 42.62.Cf; 81.65.Cf; 68.35.Np     

Ultraviolet laser surface treatment for biomedical applications of β titanium alloys: morphological and structural characterization

To improve the surface properties of titanium alloys developed for biomedical applications we have recently suggested a methodology involving laser-assisted nanostructuration. This strategy would benefit from superficial laser heat treatment since laser annealing displays many advantages as compared to the conventional methods: high resolution, high operating speed, low cost and retaining the initial bulk properties. Therefore, this paper reports results concerning the laser treatment of titanium alloys under vacuum. Our interest has been focused on the excimer laser single-pulse irradiation (=248 nm) of a model titanium alloy (Ti6.8Mo4.5Fe1.5Al). The threshold laser fluences corresponding to transus, melting and ablation temperatures as well as the resulting modification depth were first approached theoretically. KrF laser annealings were then carried out in vacuum, varying the fluence conditions from submelting heating to ablation regimes. Subsequent atomic force microscopy and scanning electron microscopy observations were performed to follow the structural and topographical modifications of as-treated specimens and were then discussed as regards the above-mentioned theoretical parameters . PACS 81.05.-t; 68.37.Ps; 68.55.Jk     

Pulsed laser coating removal by detachment and ejection

The pulsed laser removal of paint coatings from substrates by detachment and ejection of the entire layer has been studied. The dependence of the removal efficiency on fluence, coating thickness and pulse duration has been examined. For weakly absorbing coatings on easily ablating substrates, efficiencies as high as 800 mcm²J^-1 have been measured, i.e., more than two orders of magnitude higher than obtainable by surface ablation. PACS 52.38.Mf; 42.62.Cf; 81.65.Cf     

Nd : YAG laser cladding of marine propeller with hastelloy C-22

Nd:YAG laser cladding with automatic wire feeding (Hastelloy C-22) has been done to increase the lifetime of marine propellers made of HBsC1. The effects of processing parameters on the quality of clad layer have been investigated and clad layers have analyzed by optical microscopy and Vickers hardness tester. The method to overcome the drop transfer problem during the wire feeding has been introduced. A cladding speed that is too fast or too slow influenced the shape of clad. The good clad layer without cracks and with low dilution has been obtained with the optimum processing parameters . PACS 81.20.Vj; 42.62.Cf; 81.20.Wk     

Single-pulse KrF laser ablation and nanopatterning in vacuum of β-titanium alloys used in biomedical applications

We report the KrF laser (248 nm) nanosecond single-pulse irradiation in vacuum of a mirror-polished surface of a model -titanium alloy. A series of single-pulse laser-annealing experiments is performed with increasing fluence adjusted to obtain conditions from submelting (a few mJ/cm²) up to intense laser ablation (30 J/cm²). The structural and morphological changes are followed by atomic force microscopy and scanning electron microscopy, as well as with other analytical techniques, and compared with the starting equilibrium mixture of the untreated sample. The results are discussed and related to the theoretical estimation of fluences corresponding to the -transus, melting, boiling and intense ablation (phase-explosion) thresholds as well as to the estimates of melt depth and lifetime evaluation as a function of fluence. By carrying out laser annealing in vacuum the surface oxidation is avoided while roughening and phase transformations are favoured within the laser-heated depth. The mirror-polished starting surface is composed of nanometric -precipitates embedded in a -matrix, whose surface is systematically 5–10 nm above the -surface. Upon laser annealing with increasing fluence the -nanocrystals transform progressively into -phase, resulting in an increasing relief. At higher fluence the conditions for increasing roughness are studied. Submicrometre roughness is expected to improve cell adhesion to -titanium alloys and hence osteointegration. PACS 81.05.-t; 68.37.Ps; 68.55.Jk     

Single-pulse two-dimensional temperature imaging in flames by degenerate four-wave mixing and polarization spectroscopy

We introduce a novel and simple experimental arrangement for single-pulse two-dimensional temperature mapping in flames by the coherent imaging techniques, degenerate four-wave mixing and polarization spectroscopy, utilizing a dual-wavelength dye laser and a diffraction grating. Each pulse of this dye laser consists of two wavelengths which were tuned to resonance with two different rotational transitions in theQ ₁ branch of theA ² –X ² (0, 0) band of the OH radical. A typical coherent imaging geometry where a sheet-shaped pump beam crossed an unfocused probe beam, was used. The two generated images of OH signal distributions were spatially separated by a diffraction grating and simultaneously detected on a single CCD chip. The two-dimensional single-pulse temperature map was extracted from these images. The precision of the methods is examined and a comparison between degenerate four-wave mixing and polarization spectroscopy is made.     

Acoustic substrate expansion in modelling dry laser cleaning of low absorbing substrates

Acoustic expressions have been derived for the thermal expansion of substrate surfaces due to irradiation by an exponential laser pulse. The result of acoustic effects on three substrates (silicon, glass and silica) with different absorptions has been calculated.It has been shown that for substrates having relatively low absorptions, like silica and glass, acoustic considerations substantially reduce thermal expansion of the substrate caused by irradiation by nanosecond laser pulses relative to a quasi-static expansion model. In particular, the expansion of the substrate occurs over a much longer time frame than when the quasi-static approximation holds. Consequently, acceleration of the substrate surface is greatly reduced and laser cleaning threshold fluences for particle removal are increased.The predictions of the model of Arnold et al. when developed for acoustic considerations give reasonable agreement with experimentally found threshold fluences for alumina particles on silica and glass substrates although it underestimates the ratio of the threshold cleaning fluences of silica and glass. This could be due to the model underestimating the contribution of surface expansion to the laser cleaning process. The influence of multiple reflections in the substrate and departure from one dimensionality in the heat conduction on the threshold fluence was found to be insignificant. Thermal contact between the particle and the substrate was also found to have little effect on laser cleaning threshold fluences. Another mechanism that may enhance surface expansion is the 3D focussing of radiation by the particles. PACS 42.62.Cf; 81.65.Cf; 42.55.Lt     

Direct writing and in-situ material processing by a laser-micromachining projection microscope

The new laser micromachining projection microscope enables a surface to be viewed at high magnification with a built-in facility for micromachining of the same surface according to a predetermined pattern. A XeCl-excimer laser forms part of the device and provides bright viewing illumination in amplified spontaneous emission as well as a small laser spot of sufficient fluence to mark the sample surface. The basic characteristics of image enhancement in the excimer amplifier are presented. The advantage of such a technique for both pattern generation, surface cleaning and mask correction is demonstrated on Al, Cu and W specimens with a spatial resolution better than 3 m.     

Transient pressure induced by laser ablation of liquid toluene: toward the understanding of laser-induced backside wet etching

To clarify the initial stage of laser-induced backside wet etching, we directly measured transient pressure upon laser ablation of toluene under KrF excimer laser irradiation by using a fast-response pressure gauge. The propagation time of the pressure peak to the gauge agreed well with the time for the shock wave to reach the gauge on time-resolved images. The peak pressure P decreased slowly with increasing distance d: P=30.4 MPa for d=100 m to P=11.1 MPa for d=1000 m at a fluence F=1.0 Jcm^-2pulse^-1. The initial pressure, estimated to be of the order of 10–200 MPa, impinges on a transparent plate and contributes to the etching. PACS 79.20.Ds; 47.40.Nm; 47.55.Bx; 42.62.Cf; 81.65.Cf     

Modelling laser cleaning of low-absorbing substrates: the effect of near-field focussing

A three-dimensional model for laser cleaning of spherical, transparent particles on low-absorbing substrates has been developed. It takes into account near-field focussing of the laser radiation by the particles. The intensity distribution under a particle was found using Mie theory together with the geometrical optics approximation. This permits the estimation of the beam width at the substrate surface and the focal distance of the radiation coming from the spherical particle. These parameters are used to find the distribution of intensity within the low-absorbing substrate from the formula for a focussed Gaussian beam. This is in contrast with most other models of laser cleaning, which assume that all absorption occurs at the surface of the substrate. The energy criterion was used to calculate the threshold fluence. The model predicts threshold fluences of the order of 10³ J/cm² for silica spheres having a diameter of the order of a micron on silica substrates, assuming adhesion by van der Waals force. As this is well above the damage threshold for silica, it effectively predicts that laser cleaning of silica spheres from silica will be impossible. For glass slides the threshold fluence is predicted to be a factor of 10^-4 times smaller than that for silica slides (about 0.1 J/cm²). This is due to the much higher absorption of glass compared to that of silica at 248 nm. PACS 42.62.Cf; 81.65.Cf     

Pulsed laser-target effects and high-resolution process diagnostics

In pulsed laser-target interaction at fluences and fluxes in a range of interest close to the air-breakdown threshold limits, the processes of heating, boiling, melting, chemical decomposition or vaporization occur quasi-explosively. Nonlinear effects due to plasma formation then largely determine the response of the materials. Investigations were carried out at various IR wavelengths (at 2.7 m, 3.8 m and 10.6 m) using a multigas laser (at more than 10 J/pulse) which could be operated as a chemical HF or DF laser and as CO₂ laser in the single-pulse mode. Studies in the repetitively pulsed mode, providing both high peak power densities and rather high average power densities simultaneously, were carried out at =10.6 m with a specially designed multi-kW CO₂ laser system. Both thermal and mechanical effects were studied on dielectric, semiconducting and metallic targets in a large fluence range from several up to hundreds of joules per cm².Valuable insight into the transient phenomena was obtained using fast optical, optronic and electronic techniques. The methods applied, including high-speed photography, videography and laser diagnostics, have proved to be particularly useful for process monitoring or control in industrial laser materials processing applications in order to improve energy transfer rates for optimization purposes.     

Micropropulsion using laser ablation

The micro-laser plasma thruster (LPT) is a new micropropulsion device that uses laser ablation to create very small thrusts (0.1–100 N) for pointing and positioning micro- and nano-satellites. In this paper, we discuss the expected performance of the LPT. For a ms-pulse device, target materials are restricted to those of low thermal conductivity, e.g. polymers. Volume ablation theory adequately describes their behavior. In a ns-pulse version, exhaust velocity can be an order of magnitude higher with correspondingly lower thrust-to-power ratio. The theory for surface absorbers describes the observed behavior. PACS 45.55.Px; 42.70.Jk; 52.50.Jm; 42.62.Cf; 42.70.Km; 78.66.Qr     

Effect of composition gradient on CuIn3Te5 single-crystal properties and micro-Raman and infrared spectroscopies

The topography and roughness evolution of surfaces etched by laser-induced backside wet etching (LIBWE) is investigated in detail. The etching of sub-m gratings with a period of 760 nm into flat surfaces by means of interfering laser beams shows a saturation of the grating depth within 20 pulses. The over-etching of already microstructured surfaces results in the change of the cross section and in reduction of the microstructure height with increasing pulse number. The decrease in height of sub-micron gratings from 125 to less than 10 nm within 15 laser pulses causes a substantial roughness reduction. The depth limitations in etching of the gratings as well as the height reduction of microstructures are the result of the influence of the surface topography to the heat flow. The more efficient heating of surface peaks in contrast to the valleys results in higher etch rates and probably causes the smooth surfaces observed in LIBWE processing. The thermal diffusion length determines the structure dimension influenced by this smoothing effect. PACS 81.65.Cf; 81.05.Je; 42.70.Ce; 42.55.Lt     

Numerical and experimental investigation of laser propulsion

Aiming at the application of laser propulsion to stratospheric flight, we propose here a repetitive propulsion system, friction-less environment and metal-free ablation in a water cannon target. A special reservoir of water is proposed to supply a definite amount of water only at the time of laser irradiation. This system is tested for the object levitated on the airflow, which is called air-slider in order to simulate friction-less environment. In order to avoid the evaporation and freezing of water in lower atmospheric pressures, we propose here an air curtain system. We have performed simulations and experiments and confirmed that a small amount of air flow can sustain the high pressure inside the water reservoir. PACS 42.62.Cf; 52.38.Mf; 07.87.+v     

Laser cleaning on Roman coins

Ancient metal objects react with moisture and environmental chemicals to form various corrosion products. Because of the unique character and high value of such objects, any cleaning procedure should guarantee minimum destructiveness. The most common treatment used is mechanical stripping, in which it is difficult to avoid surface damage when employed. Lasers are currently being tested for a wide range of conservation applications. Since they are highly controllable and can be selectively applied, lasers can be used to achieve more effective and safer cleaning of archaeological artifacts and protect their surface details. The basic criterion that motivated us to use lasers to clean Roman coins was the requirement of pulsed emission, in order to minimize heat-induced damages. In fact, the laser interaction with the coins has to be short enough, to produce a fast removal of the encrustation, avoiding heat conduction into the substrate. The cleaning effects of three lasers operating at different wavelengths, namely a TEA CO₂ laser emitting at 10.6 m, an Er:YAG laser at 2.94 m, and a 2-Nd:YAG laser at 532 nm have been compared on corroded Romans coins and various atomic and nuclear techniques have also been applied to evaluate the efficiency of the applied procedure. PACS 42.55.-f; 42.55.Ah; 81.05.Bx.     

Interface mixing in Co/Cu: in situ electrical conductivity measurements on pulsed-laser-deposited multilayers

By providing the experimental possibility to monitor the electrical conductivity of metallic multilayers prepared by laser ablation during their growth in situ, information can be obtained on interface mixing effects. This is demonstrated for (5 nmCo/5 nmCu)_n multilayers ablated by a UHV–193 nm ArF–Pulsed Laser Deposition (PLD) system. Varying the laser fluence from 4.2 Jcm^-2 to 12.3 Jcm^-2 allows a corresponding variation of the fraction f of energetic particles within the ablated plume as well as of their average energy. As a result of such a bombardment-assisted deposition, mixing effects occur for both growth sequences, Co on top of Cu as well as Cu on top of Co. As expected, these mixing effects are most pronounced for high laser fluences and significantly decrease for fluences close to the ablation threshold. Comparison to TRIDYN Monte Carlo simulations allow one to extract estimates for the fraction f of energetic particles as well as for their average energy. PACS 73.40.Jn; 73.61.At; 75.47.Np; 81.15.Fg     

Effective length of filaments measurement using backscattered fluorescence from nitrogen molecules

The effective length of self-induced long (100 meters) plasma filaments, generated by intense femtosecond near IR laser pulses in air, was measured remotely using a lidar technique. This technique is based on detecting the backscattered fluorescence signal from nitrogen molecules excited along the intense laser pulse propagation path. This opens up the possibility of measuring remotely long filaments extending over hundreds of meters in the atmosphere. PACS 42.65.Jx; 42.68.Ay; 42.68.Wt     

Femtosecond pulsed laser ablation of 3CSiC thin film on silicon

A femtosecond pulsed Ti:sapphire laser (pulse width=120 fs, wavelength=800 nm, repetition rate=1 kHz) was employed to perform laser ablation of 1-m-thick silicon carbide (3CSiC) films grown on silicon substrates. The threshold fluence and ablation rate, useful for the micromachining of the 3CSiC films, were experimentally determined. The material removal mechanisms vary depending on the applied energy fluence. At high laser fluence, a thermally dominated process such as melting, boiling and vaporizing of single-crystal SiC occurs. At low laser fluence, the ablation is a defect-activation process via incubation, defect accumulation, formation of nanoparticles and final vaporization of boundaries. The defect-activation process reduces the ablation threshold fluence and enhances lateral and vertical precision as compared to the thermally dominated mechanism. Helium, as an assistant gas, plays a major role in improving the processing quality and ablation rate of SiC thin films due to its inertness and high first ionization energy. PACS 79.20.Ds; 42.62.Cf; 42.70.Qs; 61.72; 61.46     

Characterization of laser-treated paper

Paper is one of the most important materials in cultural heritage given its extensive use as the data carrier for religious, artistic and scientific records. For both aesthetic and conservation reasons, cleaning of these materials is often needed. Current paper cleaning methods using conventional means are not always sufficient, e.g. for the local cleaning of paper in the vicinity of sensitive media. In this respect a ns-pulse laser provides a valuable tool for solving difficult cleaning problems. The influence of various laser wavelengths (355 nm, 532 nm, and 1064 nm) and the ageing status of modern paper test systems were studied. Colorimetric measurements, the determination of the average molecular mass of cellulose, and chemiluminescence analysis proved to be useful for the characterization of the laser-treated paper. Treatment with green laser light at =532 nm below the paper ablation threshold fluence gave the most promising results on pure papers, with no discolouration and no other visible alteration, nor detectable chemical changes. PACS 06.60.J; 78.60.Ps; 81.65.Cf